Reactions of a phosphido-bridged unsymmetrical diiron complex (η⁵-C₅Me₅)Fe₂ (CO)₄(μ-CO)(μ-PPh₂) with various alkynes

A phosphido-bridged unsymmetrical diiron complex (η⁵-C₅Me₅)Fe₂ (CO)₄(μ-CO)(μ-PPh₂) (1) was synthesized by a new convenient method; photo-dissociation of a CO ligand from (η⁵-C₅Me₅)Fe₂ (CO)₆(μ-PPh₂) (2) that was prepared by the reaction of Li[Fe(CO)₄PPh₂] with (η⁵-C₅Me₅)Fe(CO)₂I. The reactivity of 1 toward various alkynes was studied. The reaction of 1 with ^tBuC≡CH gave a 1:1 mixture of two isomeric complexes (η⁵-C₅Me₅)Fe₂ (CO)₃(μ-PPh₂)[μ-CH≡C(^tBu) C(O)] (3) containing a ketoalkenyl ligand. The reactions of 1 with other terminal alkynes RC≡CH (R=H, CO₂Me, Ph) afforded complexes incorporating one or two molecules of alkynes and a carbonyl group. The principal products were dinuclear complexes bridged by a new phosphinoketoalkenyl ligand, (η⁵-C₅ Me₅)Fe₂(CO)₃(μ-CO)[μ- CR¹=CR²C(O)PPh₂] (4a: R¹=H, R²=H; 4b: R¹=CO₂Me, R²=H; 4c: R¹=H, R²=Ph). In the cases of alkynes RC=CH (R=H, CO₂Me), dinuclear complexes having a new ligand composed of two molecules of alkynes, a carbonyl group, and a phosphido group; i.e. (η⁵-C₅Me₅) Fe₂(CO)₃[μ-CRCHCHCRC(O)PPh₂] (5a: R=H; 5b: R=CO₂Me), were also obtained. In all cases, mononuclear complexes, (η⁵-C₅Me₅) Fe(CO)[CR¹=CR²C(O)PPh₂] (6a: R¹=H, R²=H; 6b: R¹=H, R²=CO₂Me; 6c: R¹=H, R²=Ph) were isolated in low yields. The structures of 1, 4c, 5b, and 6a were confirmed by X-ray crystallography. The detailed structures of the products and plausible reaction mechanisms are discussed.